1. Field of the Invention
This invention relates to a spark plug which has an electrode metal made from a heat-and erosion-resistant nickel alloy whose front end has a noble metal tip made of iridium or ruthenium.
2. Description of Prior Art
In a spark plug electrode for an internal combustion engine, a noble metal tip is used which has been made of iridium or ruthenium since they are superior in spark-erosion to other noble metals such as platinum or the like. This is because iridium and ruthenium have a higher melting point (2447.degree. C., 2310.degree. C.) than that of platinum by 600.degree..about.700.degree. C.
However, iridium and ruthenium are particularly vulnerable to an oxidation-based evaporation at high temperature so as to be quickly corroded when the temperature exceeds a critical point. That is to say, wear of the noble metal tip is accelerated at the critical temperature when made of iridium or ruthenium.
In order to avoid the rapid wear of the noble metal tip, Japanese Patent Application No. 4-350 introduces a center electrode 100 for a spark plug as shown in FIG. 6. In the center electrode 100, a recess 102 is provided on a front end of an electrode metal 101, and a noble metal tip 103 is fixedly placed in the recess 102. The electrode metal 101 clads a heat-conductive core 104 whose front end 104a is located near a front end 103a of the noble metal tip 103. The heat-conductive core 104 works to draw a considerable amount of heat from the noble metal tip 103 so as to keep the temperature of the tip 103 from exceedingly rising.
In this instance, the electrode metal 101 is made of Inconel 600 so as to satisfactorily resist a thermal stress caused by a thermal expansional difference between the noble metal tip 103 and the front end of an electrode metal 101. The Inconel 600 has a good physical strength at high temperature, but not a sufficient thermal conductivity to draw the heat from the noble metal tip 103.
Therefore, it is an object of the invention to provide a spark plug which is capable of maintaining the temperature of a noble metal tip relatively low so as to significantly reduce the wear to which noble metal tip is subjected.